Describe the structure and bonding that contributes to a functional protein.

Translation of an mRNA transcript by ribosomes produces the two-dimensional primary structure – a linear sequence of amino acids held together by strong peptide (covalent) bonds. This linear sequence typically folds into one of two structural motifs – secondary structures. The right handed alpha helix is a tight coil of amino acids stabilised by hydrogen bonds between each amino acid and the amino acid directly above it in the helix. The beta sheet consists of extended chains of amino acids (beta strands) running alongside each other either in parallel or anti-parallel and held together by hydrogen bonds.The tertiary structure is a combination of these secondary structures and is held together by a variety of different bonds. The weakest bonds are van der Waals forces which act most significantly between hydrophobic regions. Hydrogen bonds are found in hydrophilic regions and act between R groups or R groups and water. Stronger bonds include ionic bonds, which act between residues of opposite charge, and disulphide (covalent) bonds, which form between cysteine residues. Many proteins also have a quaternary structure formed of multiple peptide chains and/or prosthetic groups.